The present invention relates to a vehicle steering control system for controlling one or more vehicle motion variables so as to achieve prescribed vehicle response characteristics by varying a manipulated quantity of a steering system, and especially to a vehicle steering control system having an internal feedback loop.
The possibility of improvement in steering characteristics of a conventional two wheel steer vehicle (in which only front wheels are steered by angular displacement of a steering wheel) is limited because of the very nature of the two wheel steer vehicle.
A conventional example of a control system for a four wheel steer vehicle is disclosed in "Gakujutsu Koenkai Maezurishu 842058" (pages 307-310) published by Society of Automotive Engineers of Japan in 1984. The control system of this example alters a rear wheel steer angle by sensing an actual yaw rate with a yaw rate sensor and feeding back the sensed value of the yaw rate so as to maintain a steady state side slip angle equal to zero.
However, this system requires a sensor for sensing a vehicle motion variable such as a yaw rate, so that the cost and the accuracy of such a motion variable sensor are troublesome. The disadvantage is further inceased when the system requires a plurality of the vehicle motion variable sensors. Furthermore, noise is superimposed on the sensor signal by vehicle vibrations, and the delay in the feedback loop is increased.